A. Technical Field
The present invention relates to a new star-shaped block copolymer and a production process therefor, wherein the star-shaped block copolymer exhibits high heat resistance despite having a low viscosity.
B. Background Art
There is a known star-shaped block copolymer that has a structure in which at least three polymer chains are radially extended from a central polyvalent mercaptan segment. This star-shaped block copolymer generally has more excellent heat resistance than simple chain polymers. Herein, the heat resistance means the cohesion or strength of polymers at high temperature, for example, the holding power at high temperature in the field of pressure sensitive adhesives. The increase of the molecular weight is an effective method to enhance the heat resistance. In the case of the simple chain polymers, however, the simple increase of the molecular weight leads to great increase of viscosity and therefore causes problems of workability. In comparison, because of the above structure, the star-shaped block copolymer can have its heat resistance enhanced without so much increase of its viscosity. Thus, the star-shaped block copolymer is favorably used for purposes to which the heat resistance is demanded, such as hot-melt pressure sensitive adhesives.
The star-shaped block copolymer is generally synthesized by a process comprising the multistep radical polymerization which involves the use of a polyvalent mercaptan having a valence of 2 to 6 as an initiator wherein the elementary steps of the multistep radical polymerization differ in polymerizable monomers as used therein (JP-A-07-179538):
Also as to the star-shaped block copolymer, in the case where higher heat resistance is demanded thereto, the above method involving the increase of the molecular weight might enhance the heat resistance in some degree, but the increase of the viscosity would be still so great that problems of workability would occur.